Solid-state lighting (SSL) is rapidly becoming a dominant lighting technology and is expected to largely displace most other conventional lighting technologies, such as incandescent and fluorescent lighting, in the not too distant future. SSL is a superior lighting technology since it employs light-emitting diodes (LEDs), which are energy efficient, long-lasting, and durable.
Although SSL technology is superior to conventional lighting technologies in many respects, one well-known problem with its use relates to the ability to dim the light produced by the LEDs. Dimming is desirable since it helps conserve energy. It also allows the ambiance of an environment to be altered. Unfortunately, because LEDs are nonlinear devices, the conventional analog dimmer switches commonly found in homes and offices to control dimming of conventional incandescent lighting sources cannot be used to directly control dimming in SSL systems.
Another approach which has proved to be useful in controlling dimming in SSL systems is pulse-width modulation (PWM) based dimming. In PWM-based dimming periodic control signals containing sequences of pulses are generated and modulated to control how often the LEDs in the SSL system conduct and thereby produce light. By reducing the durations (i.e., “widths”) of the pulses in the control signals, the ratio of time in which the LEDs are configured in a conducting/light-producing state to the time they are configured in a non-conducting/no-light-producing state (this ratio often being referred to as the “duty cycle”) is reduced. This duty cycle reduction is perceived by the human eye and brain as a dimming of the light.
PWM-based dimming can be a useful approach to dimming LEDs in SSL systems. However, under some circumstances it can undesirably produce flicker and electromagnetic interference (EMI). Flicker, is particularly perceptible when the duty cycles of the control signal are low, and it can cause or induce headaches, blurred vision, eyestrain and other health-related discomforts and disorders. EMI can also be problematic since it can interfere with reception in radios and degrade the performance of other electronic devices. Additionally, PWM-based dimming is capable of dimming only over a limited dynamic range. The ability to dim over a wide dynamic range and to very low dim levels requires the ability to adjust the PWM control signals so that they have very narrow pulses. However, generating very narrow pulses and designing driver circuitry capable of responding to very narrow pulses is difficult and in some circumstances even impracticable.